1. Field of the Invention
The present invention relates to an instantaneous phase-shift interferometer. In particular, the present invention relates to an instantaneous phase-shift interferometer being proposed to improve accuracy, enabling a user to easily pre-measure (calibrate) a bias, amplitude, and amount of phase shift between a plurality of phase-shifted interference fringe images obtained by, for example, a plurality of different image capturers or in different image capture regions of a single image capturer.
2. Description of Related Art
An interferometer analyzing a phase of interference fringes produced by light reflected from a reference surface and light reflected from a measured object is a device capable of measuring a shape of the measured object with a high degree of accuracy using a wavelength of the light as a measuring stick. A representative technique of analyzing, with a high degree of accuracy, the phase of the interference fringes obtained by the interferometer is a phase-shift interferometer which shifts the phase of the interference fringes and captures and analyzes a plurality of images. Among such phase-shift interferometers, an instantaneous phase-shift interferometer uses a plurality of interference optical paths and a camera to simultaneously measure a plurality of phase-shifted interference fringes required for analysis. The instantaneous phase-shift interferometer is an extremely practical interferometer that is capable of measuring a shape of a measured object in a vibration environment such as an assembly plant, to which conventional phase-shift interferometers are ill suited.
In the instantaneous phase-shift interferometer, errors in analysis of the interference fringe phase are generated by variation in bias and amplitude between the plurality of interference fringe images captured by different cameras, and a difference between a defined value in calculation of a phase shift amount and an effective value achieved optically. Accordingly, optical parameters characteristic of each individual interferometer (i.e., bias, amplitude, and amount of phase shift of interference fringes obtained by each camera) are measured ahead of time, and when the actual measured object is measured, a highly accurate measurement cannot be obtained unless calculations are performed with such optical parameters taken into consideration.
The applicant has disclosed a method of measuring these optical parameters in Japanese Patent Laid-open Publication Nos. 2002-13907 (FIGS. 1 and 3) and 2002-13919 (FIGS. 1 and 2). In these documents, in an instantaneous phase-shift interferometer shown in FIG. 1, a measured object (described as a detected surface 7 in both documents) is shifted Δdi in an optical axis direction with respect to a reference surface 5, as shown in FIG. 2, and a plurality of interference fringes are imaged. Then, by performing data processing using a formula noted in both documents, for example, the optical parameters of the instantaneous phase-shift interferometer can be calculated. In the drawings, 1 is a laser light source, 2 is a lens, 3 is a beam splitter, 4 is a collimator lens, 6 and 8 are ¼ wave plates, 9 is a three-way (splitting) prism, 10 to 12 are polarizing plates, and 13 to 15 are image capture devices.
In order to achieve the previously proposed method, the measured object 7 must be accurately shifted in the optical axis direction with respect to the reference surface 5 of the instantaneous phase-shift interferometer. In order to do this, a costly, extremely high-accuracy scanning stage must be prepared and the measured object 7 must be installed on the stage so as to not move. In addition, vibration in a measurement environment during measurement of the optical parameters may induce a relative shift error of the measured object 7 with respect to the reference surface 5, and therefore an accurate measurement environment eliminating vibration must be prepared.
Because the errors are fixed and characteristic of each individual interferometer, measurement of the optical parameters should theoretically be performed only once after assembly and adjustment of the interferometer. However, in reality, the optical parameters change due to changes to optical devices configuring the interferometer over time, deformation of an interferometer casing holding the optical devices, and the like. Accordingly, measurement must be performed periodically and values updated in order to maintain a high level of performance. Bringing the interferometer to an environment where the measured object 7 can be shifted accurately, and accurately shifting the measured object 7 to measure the optical parameters each time this is done require a great deal of effort.
Specifically, the conventional technique proposed to improve accuracy of an instantaneous phase-shift interferometer requires a special device and special measurement environment, and poses extreme difficulties to a user performing measurement periodically.